1. Field of the Invention
This invention relates to a straddle, and in particular a straddle for use in selectively isolating a section of tubing. The invention also relates to a method of isolating a section of tubing.
2. Description of the Related Art
In the oil and gas exploration and production industries, subsurface hydrocarbon-bearing formations are accessed via casing-lined wellbores. The lower section of a bore, which intersects the hydrocarbon-bearing formation, is typically lined with perforated “liner”, oil and gas flowing into the bore through the perforations. The location of the perforations is predetermined on the basis of surveys, to ensure that only selected formations are in fluid communication with the bore. Over the life of a well it may occur that the properties of particular formations change, for example the pressure in a formation may fall, or a formation may begin to produce any unacceptably high volume of water. In these circumstances it is known to run straddles into the liner, these straddles being sections of tubing with sealing arrangements at either end. A straddle may be located within the section of liner intersecting the problem formation, and the seals then set to isolate the section of liner between the seals. However, existing straddles are problematic to set, and the requirement to accommodate the seals and a seal setting mechanism result in a significant loss in bore cross section, which reduces the production capacity of the well and also makes it more difficult to access the section of well beyond the straddle.